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If it was losing leaves right away, it may have arrived suffering some stress.
Your watering sounds correct, but make sure the soil is not draining to quickly.
Look carefully for signs of bugs or insects.
Here is a link that may help your determine a cause.

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

Brown spots and edges in houseplants typically are caused by watering issues. I would first check to see if the plant is rootbound, which could prevent the roots from taking up water. That is most likely the cause. Additionally, it could be a fungus or even pests. Neem oil can help for both of these. For other reasons why plants get brown edges on leaves, this article will help: https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm

It is hard to say what actually caused the plant to turn brown.

It could have been stressed from leaving the garden center, fungus issue, insects, over watering, sun scorch to name a few reasons.

Generally a garden center will guarantee a plant for the first year. I would see if they would replace the tree.

Here is a link to refresh you on the care requirements.

https://www.gardeningknowhow.com/ornamental/trees/umbrella-pine/umbrella-pine-planting.htm

This is the plant actually blooming.
This is fairly rare for this plant so enjoy and great job!

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

Pruning will not keep the roots from growing. Your plant may be to be repotted once a year.

https://www.gardeningknowhow.com/houseplants/schefflera/schefflera-plant-care.htm

I would do a Google search for granular forms of Sterilization chemicals. You should be able to find a source that can ship direct to you.
Here is a link about this procedure, though it can be a difficult and time specific.